Materials Week '97: Wednesday PM Session

Focusing on physical metallurgy and materials, Materials Week '97, which incorporates the TMS Fall Meeting, features a wide array of technical symposia sponsored by The Minerals, Metals & Materials Society (TMS) and ASM International. The meeting will be held September 14-18 in Indianapolis, Indiana. The following session will be held Wednesday afternoon, September 17.

NON-LINEAR EFFECTS IN MATERIALS SCIENCE: Session IV: Surfaces and Interfaces

Session Chair: F.G. Yost, Sandia National Laboratories, Albuquerque, NM 87185

STICK SLIP RELATIVE MOTION BETWEEN CONTACT BODIES: James C.M. Li, Materials Science Program, Department of Mechanical Engineering, University of Rochester, Rochester, NY 14627

The stick/slip phenomena are described and its source analyzed. High speed data acquisition for velocity variations during slip and atomic force microscopy observations for the impression profile during stick provide sufficient clues to reveal the origin of non-uniform relative motion. The roles played by the mechanical properties of the materials and stiffness of the loading system are emphasized. Quantitative understanding of the of horizontal and normal force variations and the time and distance periods between sticking is possible in some situations such as scratching. Some environmental effects are described and analyzed also. This work was supported by NSF through DMR9623808 monitored by Dr. Bruce MacDonald.

2:30 pm INVITED

ROUGH SURFACES GENERATED BY NON-LINEAR TRANSPORT: J. Adin Mann, Jr.*, Wojbor A. Woyczynski, Case Western Reserve University, Dept. of Chemical Engineering* and Department of Statistics, Cleveland, OH 44106

Kardar, Parisi and Zhang (KPZ) in their 1986 paper provided a non-linear governing equation for the instability of growth of a solid-fluid interface. Their results and contributions from other authors have provided a theory for understanding interfacial instability in many situations ranging from the melt/solid interface to the roughness of the ocean floor and includes chemical vapor deposition (CVD) processing. The physical reasoning that KPZ use in interpreting the constitutive coefficients that appear in their govern equation is flawed. We show that the KPZ equation can be obtained for CVD by a mass balance at the interface that includes chemical reactions at the surface that occur as part of the growth mechanism. The assumption of an overly simplified reaction mechanism and that the flux from the vapor phase is ballistic leads to the KPZ equation where now each constitutive coefficient is well defined. Moreover, extensions to KPZ become obvious and a rich set of non-linear governing equations result. KPZ assumes that the surface transport is pure diffusion. However, under certain conditions we know that trapping can occur in such a way that the diffusion mechanism must be generalized. We are exploring an approach using a "fractional order" Laplacian operator to augment the Laplacian operator of the diffusion reaction equation. The physical ideas of this extension will be discussed.

3:00 pm INVITED

THE MORPHOLOGICAL STABILITY OF ALLOY THIN FILMS: J.E. Guyer, P.W. Voorhees, Department of Materials Science and Engineering, Northwestern University, Evanston, IL 60208

We consider the stability of a growing alloy thin film to both changes in the morphology of the film surface and the compositional uniformity of the film. The instability is a result of the elastic stress generated by the lattice mismatch between the film and substrate, and the composition modulations in the film. The composition modulations are due to the morphological instability in the surface of the growing film; there is no bulk diffusion in the film. We find that for some lattice-matched m-v alloys the composition modulations themselves can induce a morphological instability in the film surface. Due to the compositionally generated stresses, the morphological instability in the film surface can be in the form of traveling surface waves. The relationship between composition modulations induced by the morphological instability of the film surface and the more classical explanation for these composition modulations, spinodal decomposition, will also be discussed. This work was supported by the NSF-MSREC under award DMR9120521.

3:30 pm BREAK

3:40 pm INVITED

ATOMIC MODELING OF CRITICAL PROCESSES IN THE NON-LINEAR CRACK TIP ZONE OF CRYSTALLINE SOLIDS: R.G. Hoagland, School of Mechanical and Materials Engineering, Washington State University, Pullman, WA 99164-2920

Because of the analytical intractabilities posed by the non-linear properties of the cores of crack tips and dislocations, elasticity offers little help in describing the details of the energetics of crack extension and dislocation emission. Atomistic modeling, however, provides a useful alternative for studying some of the key features of crack tip behavior and the factors that distinguish a brittle material from an intrinsically tough material. In this paper, we describe some of the non-linear properties within crack tip and dislocation cores that derive from such calculations. We also examine the energetics of crack tip evolution and show that the Peierls-like criterion posed by Rice et al. consistently predicts the behaviors of atomic models of crack tips in terms of defining the competition between dislocation emission and crack extension. This work was supported by the Division of Materials Science, Office of Basic Energy Sciences, U.S. Department of Energy through Grant DEFG-6-87ER45287.

4:10 pm

OSTWALD RIPENING IN ELASTICALLY STRESSED SOLIDS: N. Akaiwa, M.E. Thompson, P.W. Voorhees, Department of Materials Science and Engineering, Northwestern University Evanston, IL 60208; National Research Institute for Metals, Japan

A two-phase microstructure can evolve in time even when the phase compositions and volume fractions are nearly at their equilibrium values. Such an evolution process is called Ostwald ripening, or coarsening. For stress-free systems, the evolution of the microstructure is driven by a decrease in the total interfacial energy of the two-phase mixture. In a wide array of two-phase solid systems, however, the ripening process is frequently accompanied by elastic stress. In these cases, ripening is driven by a decrease in the sum of the elastic and interfacial energies. This change can give rise to qualitatively new phenomena, such as inverse Ostwald ripening wherein small particles grow at the expense of large particles, large-scale particle migration and particle shape bifurcations. A discussion of these phenomena as well as the results of recent calculations on the kinetics of ripening using large numbers of elastically and diffusionally interacting particles will be presented. This work was supported by the NSF under award DMR-9322687.

4:40 pm

NON-LINEAR SOLUTE SEGREGATION DURING IRRADIATION: E.P. Simonen, S.M. Bruemmer, Pacific Northwest National Laboratory, P.O. Box 999 MS P8-5, Richland, WA 99352

Radiation-induced segregation of solute to grain boundaries occurs during irradiation of alloys because of discriminating interaction between point-defect flow and solute. Segregation is predicted from simultaneous solutions to transport equations for point defects and solute. Inverse-Kirkendall diffusion in irradiated stainless steel is an example of nonlinear segregation phenomena. Solutions to five rate equations, two for defects and three for solute species, are solved. Stable solutions result and are in agreement with measured composition profiles near irradiated grain boundaries. Point defect concentrations are controlled by production rates, mutual annihilation rates and annihilation by migration to grain boundaries. The mathematical formalism follows that of the well-known preditor-prey problem. This work was supported by the Materials Sciences Branch, BES,U.S. Department of Energy, under Contract DE-AC06-76RLO 1830.

5:00 pm

NON-LINEAR CORROSION INTERACTIONS AT TIPS OF STRESS CORROSION CRACKS: E.P. Simonen, C.H. Windisch and R.H. Jones, Pacific Northwest National Laboratory, P.O. Box 999 MS P8-15, Richland, WA 99352

Aqueous corrosion of metals under stress can result in stress corrosion cracking Corrosion at the crack tip is influenced by production of corrosion products chemical reactions and transport between the occluded crack environment and the bulk aqueous solution. The production and destruction of aqueous species are calculated by numerically solving a set of non-linear equations as a function of time and position inside a crack. Stable solutions are obtained that provide estimates of crack tip chemistries during stress corrosion cracking. Multiple ionic species are accounted for as well as distinct reactions at crack walls and crack tips with unique material chemistry. Crack chemistries in Ni, Fe and stainless steel have been modeled and compared with measured cracking responses. This work was supported by the Materials Sciences Branch, BES, U.S. Department of Energy, under Contract DE-AC06-76RLO 1830.

5:20 pm

APPLICATION OF FUZZY CLUSTERING IN THE ANALYSIS OF MISORIENTATION CRYSTALLOGRAPHY: Mary Ann Egan*, P. Krishnamoorthy*, Krishna Rajan, Departments of Computer Science* and Materials Science, Renssalaer Polytechnic Institute, Troy, NY 12180-590

The clustering of crystallographic orientation data has long been studied especially in the field of texture analysis. More recently, the concept of mapping misorientation information has been developed using the fonnalism of Rodrigues-Frank vectors. In this presentation we utilize a non-linear analysis termed fuzzy clustering to study misorientation data in Rodrigues-Frank space. Fuzzy clustering is a clustering mechanism that allows for ambiguity in the data and provides the corresponding degrees of support. In this presentation, we will describe fuzzy clustering techniques and the usefulness of this method. Some results are presented in the framework of determining a probability distribution function in Rodrigues-Frank space.